Characteristics of sediment dynamics following large-scale sediment supply events in mountain watersheds in Japan

Abstract

Large-scale sediment supply events (LSEs) cause not only a sudden increase in sediment discharge immediately after their occurrence, but also a long-term condition of elevated sediment yield. In Japan, many LSEs, triggered by heavy rainfall or earthquakes have created high risks for sediment-related disaster. However, the difference between sediment dynamics following a LSE triggered by rainfall and one triggered by an earthquake is unclear, as the sediment dynamics due to rainfall and those due to earthquakes have not been compared using consistent methodology. In this study, we estimated the volume of sediment supply VL (m3), time series of the volume of sediment yield VY (m3year−1) and the non-dimensional parameter NVY as estimated by dividing VL by the catchment area and annual maximum 3-day rainfall amount using the same methodology for 16 events in eight study basins in Japan. We used annual maximum 3-day rainfall amounts as a surrogate for discharge. Using these parameters, we estimated the period over which the effect of a LSE continues (PLS); sediment retention time (residence time, Tr1), and time for active sediment yield to recover to the background level (recovery time, Tr2a). In addition, we estimated the recovery time (Tr2b) based on when the maximum of NVY immediately after the LSE decreased to a certain extent. This calculation allowed us to estimate the recovery time even when background level was unknown. Both residence and recovery times ranged from <1 year to 19 years, with recovery time tending to be shorter than residence time. We found that Tr2b can be used as a substitute for Tr2a. No differences in PLS between rainfall-induced and earthquake-induced events were apparent, although VL differed markedly between event types. This discrepancy may be due to the difference in sediment connectivity between hillslopes and river channels. VY during the first and second year after a LSE associated with an earthquake is likely to be greater than that after a rainfall-associated event.